Various hydrometers are known for measuring the specific gravity of liquids. However, to date, none of these devices provides for inexpensive, convenient and yet highly accurate comparative field testing of specific gravity. Such field tests may be quite useful to military personnel for promptly determining if fuel taken from abandoned or captured motor vehicles or fuel supplies is suitable for use.
To be truly accurate, hydrometers must be temperature compensated. This is particularly important in the field where temperatures may vary from subarctic to tropical. To date, the majority of temperature compensating hydrometers have been intended for use in the laboratory or other controlled environment. They typically employ intricate and often delicate instrumentation, including thermometers and needle gauges, and are largely unsuited for the rough handling and dropping which may occur in the field. For example, glass thermometers are easily broken if dropped or handled roughly. Needles, shafts and other mechanisms may expand and contract under temperature extremes. As a result, such mechanisms can stick and further inaccuracies are introduced into the instrument.
One previous device attempts to achieve temperature compensation by employing a cell that is filled with a reference liquid. However, that device requires rather elaborate techniques to compensate for the weight, and therefore the density of the cell wall. Moreover, this device provides only an analog measure of specific gravity which must be read and analyzed. It does not provide a quick, comparative test that immediately tells even unsophisticated operators whether the test liquid is above a minimum required specific gravity.
Virtually the only known devices for providing comparative testing of specific gravity utilize styrofoam balls which float if the specific gravity of the test liquid is above that of the balls, but sink if the specific gravity of the liquid is below the level of the balls. Such devices may be acceptable for certain uses, such as the home testing of radiator and battery fluids. However, they typically provide no temperature compensation. The balls are filled with air which contracts or expands in response to the temperature of the test liquid. The specific gravity of the balls does not change proportionately with the specific gravity of the liquid and inaccurate measurements may be provided; for example, a ball may sink at one temperature and float at a second temperature. Such devices are therefore impractical when a highly accurate indication of specific gravity is required.